The movements associated with the contractile cycle of actin and myosin result from structural changes in the myosin S1 that accompany the binding and hydrolysis of ATP and rebinding of actin. The goal of my research proposal is to use site-directed mutagenesis and protein crystallography to study the structure of catalytic and structural variants of the Dictyostelium myosin S1Dc and S1 proteins: 1. Generate mutants in catalytically important residues that have been implicated in the ATPase activity of myosin to differentiate between two proposed mechanisms of catalysis for the myosin protein. 2. Generate mutants that will trap the protein in novel conformational states that are assumed during the acto-myosin contractile cycle by the introduction of covalent cross-links into the newly introduced mutations or by incorporation of mutant residues that restrict the mobility of the polypeptide chain. 3. Generate mutations at residues in the Dictyostelium myosin protein that are homologous to mutations seen in the S1 head region of the human Beta- cardiac muscle myosin result in hypertrophic cardiomyopathy, a genetic disease characterized by left ventricle, hypertrophy to study the structural basis of this disease.